Condensation on surfaces between compartments in multi-compartment refrigerators has been a problem for many years. The problem has been particularly acute on the external mullion between the freezer compartment and the fresh food compartment, each of which has an external door. Condensation is also a problem in refrigerators where the access to the freezer is inside the food compartment, but is not as severe as with two external doors.
The low temperature requirements in the freezer results in a tendency for the external surface adjacent the freezer to sweat. The surface just below the freezer door and above the fresh food door is the major area for sweating since it is influenced by the cold temperatures of both the freezer compartment and the fresh food compartment. This surface just below the freezer door and above the fresh food door is referred to as the mullion.
The problem has existed in vapor compression electrically powered refrigerators and also gas absorption type refrigerators which may be either electrically or gas energized. The problem is essentially the same regardless of the refrigeration system.
Sweating involves a psychrometric principle. When the temperature drops below the dew point of ambient air, water vapor condenses on colder surfaces. The "ambient air temperature" refers to the temperature of the air surrounding the surface and can more correctly be referred to as the dry bulb temperature. The amount of temperature drop required to cause sweating depends on the humidity ratio of the ambient air. The higher the humidity, the greater the potential for sweating because the surface temperature does not have to drop as far to condense moisture out of the air.
Many things have been proposed and/or tried to minimize the condensation problem. Some of the most common solutions include adding insulation between the inner and outer shells of the refrigerator cabinet, adding fins on the mullion to increase external surface area, heat tape, electric heaters, reducing the cooling performance for the freezer compartment, adding a loop of condenser tubing behind the mullion, and providing circulation of heated air across the problem surface.
U.S. Pat. No. 1,992,011 to Knight discloses a refrigerator cabinet which includes two hermetically sided tubes which are filled with liquid refrigerant or other volatile liquid and which are arranged along the length of the mullion and cabinet walls to transfer heat to the mullion to maintain its temperature above the dew point of the surrounding air. This system uses the heat from the outside surface of the refrigerator which is substantially at the temperature of the surrounding air.
Some examples of prior art attempts to control condensation by disposing a loop of the refrigeration condenser liquid line around the cabinet door openings include U.S. Pat. Nos. 2,135,091; 3,572,051; 3,984,223 and 4,192,149. Adding such a condenser loop causes difficult fabrication problems, especially for gas adsorption type systems which require welded steel tubing that must be properly sloped to insure gravity drainage. A condenser loop in the mullion creates an unwiedly construction and is almost impossible to replace in the field.
Electric heaters to minimize condensation are disclosed in U.S. Pat. Nos. 3,859,502 and 3,939,666. The use of electric heaters or heat tape is not available for some refrigerators such as refrigerators which are powered by propane or natural gas when electricity may not be available as in some recreational vehicles and remotely located homes.
A system for minimizing condensation is needed that is inexpensive, maintenance free, and which does not require electrical energy.